Hubbards cereals can contain more than double the sugar listed on the label, independent tests have found.

Three mueslis were tested by two different laboratories and all samples were found to have significantly more sugar than is declared on the packaging.

The levels were between 38 and 120 per cent higher than on the label - well outside the accepted range.

Double toasted Golden Syrup & Cranberry muesli claims to have 13.8g of sugar per 100g. However, four different batches were tested and found to have an average of 100 per cent more than that. This means a 600g box would contain an extra 18 teaspoons of sugar.

The tests were done by Assure Quality and Eurofins, possibly ordered by a rival, and the results were supplied to the Herald on Sunday. The newspaper does not have corresponding figures for other cereal brands.

Hubbards, founded by former Auckland mayor Dick Hubbard, markets its cereals as healthy and proudly New Zealand-made.

Chief executive Rob White said the cereal maker was surprised at the results. "We take our nutritional claims very seriously."

White said most of the sugar found in mueslis came from the dried-fruit component. A sample that had a disproportionate amount of fruit would throw up a very different result, he said.

"Mueslis are a complex mix of products: cereal, seeds, fruits, nuts. The fruit component is dried fruit and those contribute the vast bulk of the sugars in muesli.

"When we get our products, we get a whole sample and grind that up. You get an average result and not a biased result.

"We're very surprised at your results. We take it very seriously and we will do repeat testing."

Food labelling is critical for diabetics and other people who suffer from certain types of blood conditions.

Diabetes NZ recommends less than 15g of sugar per 100g.

Apricot and Papaya muesli was found to have 18.5g - more than double the 8.9g that marketing material claims.

Testing showed 5 Fruits & Honey natural muesli also had sugar levels up to 100 per cent higher than advertised.